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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Non-toxic concentrations of cyanide can be biodegraded, both aerobically and anaerobically.  Degradation of cyanides by bacteria in sewage treatment plants depends on the availability of (micro-)organisms adapted to the presence of cyanide for about two weeks.  Sudden high levels of cyanide in these sewage plants may lead to a loss of viability, while fully adapted sludge may tolerate and degrade concentrations up to 100 to 150 mg CN/liter with a high degree of efficiency.   The demonstration of ultimate biodegradability under aerobic conditions with adapted inoculum suggests that the cyanides are inherently biodegradable, even though inherent biodegradability has not been shown by the fulfillment of specifc criteria.  

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

There are numerous reports of aerobic and anaerobic/anoxic micro-organisms and treatment systems able to degrade cyanide, as reviewed in ECETOC’s JACC report: Cyanides of Hydrogen, Sodium and Potassium, and Acetone Cyanohydrin (CAS No. 74-90-8, 143-33-9, 151-50-8 and 75-86-5), 2007. No. 53: 106-122, 324. A variety of enzymatic pathways for bacterial cyanide degradation have been described from aerobic organisms, including hydrolytic pathways leading to formamide, or formate plus ammonia and direct formation of bicarbonate plus ammonia via cyanide oxidation by what is described as a dioxygenase (Fallon et al., 1991). After a week of continuous feed with low levels of cyanide ion (2 mg/liter), normal metabolic processes of organisms in municipal wastewater resumed after initial inhibition (Gurham, 1955). After adequate adaptation, 99% removal of cyanide was achieved at a feed concentration of 100 mg CN/liter. Significant cyanide removal was still observed at 200 mg CN/liter. Howe (1965) reported several techniques for the bio-destruction of cyanide waste. In an experiment with HCN being degraded by activated sludge, which had been acclimatised for two weeks, 99.8% removal was observed at 35 mg CN/liter, 98.4% removal at 50 mg/liter, 90% removal at 75 mg/liter and more than 60% removal at 180 mg/liter.